Bacteria are small, but not nearly so small as viruses. They are single - celled organisms. Bacteria are prokaryotes , which means that they do not have a cell nucleus or other membrane - bound organelles. They contain a single chromosome with double - stranded DNA. Some bacteria are saprophytic, and others are parasites. Some survive for years under adverse environmental conditions. Traditionally, they are divided into one of two groups based on their staining characteristics when exposed to the ‘ gram stain. ’ Bacteria that stain blue are said to be gram - positive; bacteria that stain pink are said to be gram - negative. Bacteria multiply rapidly in infected organisms, but can be eliminated by the host’s immunological system. Rickettsiae are small, gram - negative, intracellular bacteria that cannot live outside the cell, so unlike many bacteria they cannot be cultured on artifi cial media. They are variable in form but often rod - shaped, have DNA, RNA, and cell walls. Due to their small size, they were once thought to be viruses or positioned somewhere between viruses and bacteria. They are found in one family (Rickettsiae) and several genera (e.g., Rickettsia , Ehrlichea , and Anaplasma ). They are natural parasites of certain arthropods (lice, fleas, ticks, mites) and mammals, and cause many important diseases in humans and animals. Spirochaetes are also gram - negative bacteria, but are motile and free - living. Table 8.2 shows some bacteria that involve wildlife and insects.
The role of insects in transmitting bacteria is quite important for some diseases, but not for others. For example, avian cholera is a very important disease of birds, affecting over 190 species around the world, and sometimes causing massive death in both wild and domestic birds. About 35 species of ducks, geese and swans are susceptible, though this is the most susceptible group, with fewer than 10 species known to be susceptible in each of the following: wading birds, gallinaceous birds, doves and pigeons, woodpeckers, shorebirds, hawks and eagles, and owls. Transmission occurs principally by inhalation of aerosols (droplets), or ingestion of contaminated food or water. However, insects such as chewing lice, poultry mites, soft ticks, and tabanid flies have been shown to harbor and transmit the bacterium. Likewise, avian tuberculosis can be found in fowl tick ( Argas persicus ) and its feces, so it is implicated in mechanical transmission, though this is thought to be of little consequence as compared to direct contact among birds and contamination of food and water. In some cases, such as with heartwater, long - distance relocation of wildlife by humans can result in movement of the disease, and the ticks that vector it effectively, because the host animals are not sufficiently screened prior to transport.
